Code Division Multiple Access (CDMA) is a spread-spectrum communication technology that has become increasingly popular in mobile wireless communications systems (e.g., digital cellular radio systems). In a CDMA system, the time and frequency domains are simultaneously shared by all users as a base station simultaneously transmits distinct information signals to multiple subscriber mobile stations over a single frequency band. CDMA systems have a number of advantages over other multiple access systems (e.g., Frequency Division Multiple Access and Time Division Multiple Access) such as increased spectral efficiency and, as discussed below, the ability to mitigate the effects of signal fading using path diversity techniques.
Prior to transmission, the base station multiplies the individual information signal intended for each of the mobile stations by a unique signature sequence, referred to as a pseudo-noise (PN) sequence. This PN sequence can be formed by multiplying a long pseudo-noise sequence with a time offset which is used to differentiate the various base stations in the network, together with a short code unique to each mobile station, for example, the Walsh codes. The multiplication of the information signal by the signature sequence spreads the spectrum of the signal by increasing the rate of transmission from the bit rate to the chip rate. The spread spectrum signals for all subscriber mobile stations are then transmitted simultaneously by the base station. Upon receipt, each mobile station de-spreads the received spread spectrum signal by multiplying the received signal by the mobile station's assigned unique signature sequence. The result is then integrated to isolate the information signal intended for the particular mobile station from the other signals intended for other mobile stations. The signals intended for the other mobile stations appear as noise. The structure and operation of CDMA systems are well known. See, e.g., Andrew J. Viterbi, CDMA: Principles of Spread Spectrum Communication, Addison-Wesley Publishing, 1995; Marvin K. Simon, Jim K. Omura, Robert A. Scholtz, and Barry K. Levitt, Spread Spectrum Communications Handbook, McGraw-Hill, Inc., 1994.
One advantage of CDMA systems over other multiple-access telecommunications systems is the ability of CDMA systems to exploit path diversity of the incoming radio-frequency (RF) signal. The CDMA signal is communicated from a transmitter to a receiver via a channel including several independent paths, referred to as "multipaths". Each multipath represents a distinct route that the information signal takes between the transmitter and receiver. The transmitted signal thus appears at the receiver as a plurality of multipath signals or "multipaths". Each multipath may arrive at the receiver with an arbitrary timing delay, and each multipath may have a different signal strength at any time due to signal fading.
CDMA systems employ "rake" receivers in mobile units and base stations to exploit this path diversity. Rake receivers estimate the timing delay introduced by each of one or more multipaths in comparison with some reference (e.g., line-of-sight delay), and then use the estimated timing delays to receive the multipaths which have the highest signal strength. A typical rake receiver includes a plurality (e.g., three to six) of rake branches or "fingers". Each finger is an independent receiver unit which assembles and demodulates one received multipath which is assigned to the finger. A rake receiver also includes a separate "searcher" which searches out different signal components of an information signal that was transmitted using the assigned signature sequence of the receiver, and detects the phases of the different signal components. The timing of each finger is controlled such that it is correlated with a particular multipath which arrived at the receiver with a slightly different delay and was found by the searcher. Thus, each finger is "assigned" to a particular multipath by controlling its timing to coincide with arrival of the multipath. The demodulated output from each finger, representing one multipath, is then combined into a high-quality output signal which combines the energy received from each multipath that was demodulated. The implementation of rake receivers is generally known for both forward and reverse CDMA channels. See, e.g., R. Price and P. E. Green, Jr., A Communication Technique for Multipath Channels, 46 Proc. Inst. Rad. Eng. 555-70 (March 1958); G. Cooper and C. McGillem, Modern Communications and Spread Spectrum, Chapter 12, McGraw-Hill, NY, 1986.
In general, rake receivers estimate the channel using a searcher having a 1/2 chip resolution (i.e., -0.25/+0.25 chip resolution), and the fingers are assigned using the same resolution. The resolution of the finger assignment creates a timing misalignment between the received signal and the pseudo-noise (PN) sequence generated locally in the finger which results in signal-to-noise ratio (SNR) degradation, or degraded Frame Error Rate (FER) performance. For example, with 1/2 chip resolution for the searcher and the finger assignment, the resulting timing misalignment of 0.25 chip causes a SNR degradation on the order of 1 dB. Although receivers typically include a delay-locked loop to correct such assignment errors, the loss due to the initial timing mis-alignment becomes significant in the dynamic environments faced by CDMA mobile stations where finger re-assignments may be performed as often as every 5 to 10 frames. The delay-locked loop, which typically requires on the order of 2 frames to correct such initial timing mis-alignments, is too slow to cause the timing mis-alignment of the initial finger assignment to have a non-negligible effect on receiver performance.
One approach to decreasing the performance problems associated with timing mis-alignments caused by the initial finger assignment is to use searchers with improved resolution to estimate the channel. For example, a searcher having 1/4 or 1/8 chip resolution could be used. However, the hardware implementation of such a high-resolution searcher would be more complex than the implementation of a 1/2 chip resolution searcher, and would not be economical or practical for the construction of CDMA mobile stations. Another approach would be to use a delay-locked loop having a fast time constant directly after the initial finger assignment, followed after a period of time by a slower time constant. However, the hardware implementation of such a delay-locked loop would also be more complex, and the initial finger assignment would still result in a significant timing mis-alignment. Thus, it would be desirable to improve the initial assignment of fingers as well as the updating of finger assignments in a rake receiver without increasing the complexity of the hardware implementation for the searcher or the delay-locked loop.